As semiconductor fabrication techniques advance, the minimum feature sizes of integrated circuits (ICs) continue to shrink. Commensurate with this size reduction, various process limitations have made IC fabrication more difficult. One area of fabrication technology in which such limitations have appeared is photolithography.
Briefly, photolithography involves selectively exposing regions of a resist coated silicon wafer to a radiation pattern, and then developing the exposed resist in order to selectively protect regions of wafer layers (e.g., regions of substrate, polysilicon, or dielectric).
An integral component of photolithographic apparatus is a "reticle" which includes a pattern corresponding to features at a layer in an IC design. Such reticle typically includes a transparent glass plate covered with a patterned light blocking material such as chromium. The reticle is placed between a radiation source producing radiation of a pre-selected wavelength and a focusing lens which may form part of a "stepper" apparatus. Placed beneath the stepper is a resist covered silicon wafer. When the radiation from the radiation source is directed onto the reticle, light passes through the glass (regions not having chromium patterns) and projects onto the resist covered silicon wafer. In this manner, an image of the reticle is transferred to the resist.
The resist (sometimes referred to as a "photoresist") is provided as a thin layer of radiation-sensitive material that is spin-coated over the entire silicon wafer surface. The resist material is classified as either positive or negative depending on how it responds to light radiation. Positive resist, when exposed to radiation becomes more soluble and is thus more easily removed in a development process. As a result, a developed positive resist contains a resist pattern corresponding to the dark regions on the reticle. Negative resist, in contrast, becomes less soluble when exposed to radiation. Consequently, a developed negative resist contains a pattern corresponding to the transparent regions of the reticle. For simplicity, the following discussion will describe only positive resists, but it should be understood that negative resists may be substituted therefor. For further information on IC fabrication and resist development methods, reference may be made to a book entitled Integrated Circuit Fabrication Technology by David J. Elliott, McGraw Hill, 1989.
One problem inherent with photolithography is the Depth Of Focus (DOF) in which the wafer topographic variation of the substrate surface may cause the projected image to exhibit some decreased resolution and other optical distortions. This focus problem of the DOF lens is especially pronounced in IC designs at the high and low plateaus of the submicron features near the wavelength of light used in the photolithography (i.e., 0.25 .mu.m and smaller).
To address the effects of varied wafer topography, a number of planarization techniques have been developed including planarization layers, reflow, and chemical-mechanical polishing. One such technique is multiple resist processing in which a thick resist underlayer is first deposited atop the substrate surface to fill the valleys for planarization thereof. Subsequently, a thin, "conformal" topcoat layer of positive-acting resist, such as polymethylmeth acrylate (PMMA), is deposited on top of the underlayer which functions as the imaging layer. Accordingly, during the exposure process, this generally planar, thin top imaging surface enables the resolution of the pattern image without the adverse resolution effects encountered with the thick resist layers from the steps in the substrate surface.
One top surface imaging technique, in particular, is the Diffusion Enhanced Silylating Resist process or silylation process in which a silicon-rich mask of silicon dioxide (SiO.sub.2) is formed on the topcoat layer to provide a more resistant etch mask than conventional bilayer techniques. Typical of these patented silylation processes are disclosed in U.S. Pat. Nos.: 4,751,170; 5,550,007; and 5,665,251. In these processes, after the relatively thick underlayer of photoresist is deposited atop the substrate surface, the underlayer is usually hard baked at temperatures ranging from about 120.degree. C. to about 180.degree. C. for about 90 seconds to about 180 seconds. This process facilitates evaporation of the organic casting solvents employed to assist deposition. As importantly, this hard bake cross-links the photoresist to decompose the photoactive compounds contained therein so that they are no longer sensitive to light. Essentially, an inert planarization cross-linked matrix is formed over the substrate surface.
Subsequently, the thin topcoat layer of photoresist is deposited over the cross-linked underlayer which typically includes photoacid generators, acid labile polymer groups and chemical amplifiers which function to facilitate pattern development upon exposure of the portions thereof to ultraviolet radiation. This topcoat layer is then typically soft baked at temperatures ranging from about 90.degree. C. to about 120.degree. C. for about 30 seconds to about 120 seconds to evaporate the organic casting solvents.
As light, typically in the ultraviolet radiation range, passes through the reticle, the acid generators form a photoacid which enhances the rate of solubilization of the polymer in the areas exposed to the radiation. Briefly, photoacids function by the photolytic formation of Lewis Acids and protonic acids. These photogenerated acids catalyze the deprotection of the acid labile polymer groups or degrade the main chain by acidolysis. In the presence of photoacid, the acid labile group is cleaved from the resin matrix to deprotect the exposed region to enable silylation thereof.
The silylation process may then commence, either by a vapor phase silylation or a liquid phase silylation technique, to incorporate silicon into the radiation exposed regions of the topcoat. Typically, in a vapor phase silylation process, the wafer is placed in a chamber for exposure to a silicon-bearing compound, such as the silylating reagent hexamethyldisilazane (HMDS). In contrast, in a liquid phase silylation process, the exposed topcoat layer is dipped in a 10% Hexamethyldisilizane and 90% Xylene (warmed to 50.degree. C. w/1 gram A-methyl perolidone) bath which displaces an active proton, and incorporates a trimethylsilyl group.
The image may then be "developed" using oxygen plasma etching. During an anisotropic Reactive Ion Etching (RIE) process, the silylated region is converted to silicon dioxide (SiO.sub.2) mask. The top surface area not protected by the SiO.sub.2 mask and its underlying material can then be etched away by the RIE etch.
While this imaging technique satisfactorily addresses many DOF issues, several other problems are inherent with this process. For example, this conventional silylation technique is relatively time consuming to perform. Since both a hard bake and a soft bake are usually required in this process, at least two bake plates are required. Thus, cycle time is increased which in turn reduces efficiency and increases manufacture cost.
Another problem associated with this technique is that during the silylation process, SiO.sub.2 is added to the exposed region. Accordingly, swelling of the silylated region may occur which often distorts the final resist pattern. In turn, the feature size of the implant or etch can ultimately be affected. Accounting for or compensating for such swelling is also difficult to implement since it varies from wafer to wafer. Therefore, this adverse side effect is not easily reproducible and is most unpredictable.
Another top surface imaging technique which addresses the Depth of Focus problem is to provide a very dark resist imaging layer in either a single or multiple resist process. Due to the optical properties of the dark resist, the penetration depth of the radiation during exposure is confined to only about 400 .ANG. into the top surface and then is absorbed by the film. Therefore, better image resolution of the lithographic pattern can be accommodated since the light will not be affected by the substrate topography. Typical of these patented processes are disclosed in U.S. Pat. No. 5,023,164.
The primary problem associated with this technique, however, is that the ability to restrict and contain the acid migration of the lithographic pattern is reduced. Although the penetration depth of the radiation is confined to the top surface, the photoacid generators present in the resist may cause excessive photoacid generation which in turn produce adverse acid migration when overexposed. For example, too deep a vertical migration of the photo acid also results in adverse lateral erosion of the pattern and the undesirable deposition of residue in the etch. During the silylation process, hence, the necessary degree of silylation is difficult to control since the depth of the silylating reagent cannot be determined. In turn, the ability to control the pattern is less defined. In contrast, too shallow a radiation penetration produces inadequate etch selectivity.